Need of Concurrent Collections in java

Last Updated : 08 Jan, 2018

As we already know Collections which is nothing but collections of Objects where we deals with the Objects using some pre-defined methods. But There are several problems which occurs when we use Collections concept in multi-threading. The problems which occurs while using Collections in Multi-threaded application:

Most of the Collections classes objects (like ArrayList, LinkedList, HashMap etc) are non-synchronized in nature i.e. multiple threads can perform on a object at a time simultaneously. Therefore objects are not thread-safe.
Very few Classes objects (like Vector, Stack, HashTable) are synchronized in nature i.e. at a time only one thread can perform on an Object. But here the problem is performance is low because at a time single thread execute an object and rest thread has to wait.
The main problem is when one thread is iterating an Collections object then if another thread cant modify the content of the object. If another thread try to modify the content of object then we will get RuntimeException saying ConcurrentModificationException.
Because of the above reason Collections classes is not suitable or we can say that good choice for Multi-threaded applications.

To overcome the above problem SUN microSystem introduced a new feature in JDK 1.5Version, which is nothing but Concurrent Collections.

// Java program to illustrate Concurrent 
// Collection need 
import java.util.*; 
class ConcurrentDemo extends Thread { 
    static ArrayList l = new ArrayList(); 
    public void run() 
    { 
        try { 
            Thread.sleep(2000); 
        } 
        catch (InterruptedException e) { 
            System.out.println("Child Thread" +  
                      " going to add element"); 
        } 
  
        // Child thread trying to add new 
        // element in the Collection object 
        l.add("D"); 
    } 
  
    public static void main(String[] args)  
                   throws InterruptedException 
    { 
        l.add("A"); 
        l.add("B"); 
        l.add("c"); 
  
        // We create a child thread that is  
        // going to modify ArrayList l. 
        ConcurrentDemo t = new ConcurrentDemo(); 
        t.start(); 
  
        // Now we iterate through the ArrayList 
        // and get exception. 
        Iterator itr = l.iterator(); 
        while (itr.hasNext()) { 
            String s = (String)itr.next(); 
            System.out.println(s); 
            Thread.sleep(6000); 
        } 
        System.out.println(l); 
    } 
} 